The invention described herein relates generally to the following conventional situation. A dentist prepares a cavity of a decayed tooth to allow its restoration by means of an inlay, on lay or crown. After the preparation has been rendered, an impression of the cavity is taken, and is ordinarily sent to a dental laboratory. Contrary to conventional techniques, the method and apparatus taught herein utilize a more recent technique, which alleviates the dental laboratory and fabricates the desired restorative piece in the dental office. In particular, the prepared cavity is registered by an electro-optic scan head. The data thus obtained can be complemented by operator input, using techniques from the CAD (Computer-Aided-Design) domain, and the final piece is fabricated with the aid of a miniature NC (numerical control) grinding machine.
In the past few years, several variations of the above theme have been proposed. To highlight a few:
The first description of a system capable of registration and reconstruction of a dental preparation appeared in a report by Bruce R. Altschuler in November '71. The subsequent final report ("An Introduction to Dental Laser Holography", AFSC report SAM-TR-73-4) cites the following concept: "A contour hologram of a prepared tooth for a crown might be used to provide the information to a computer necessary for machining a piece of gold into a dental crown". The same investigator has pursued this concept, and we find further reference in "Laser Holography in Dentistry" by John M. Young and Bruce R. Altschuler, J Prostet. Dent. Aug. 77 pp. 216-225. Inlays are mentioned in this article, and it is reported that prototypes of a system employing a Laser holographic stereocomparator are in use at the Brooks USAF base.
A similar method was described a few years later by the French team of Dr. Francois Duret et al (European Published Application No. 040 165). In line with the advanced state of the art in electronic devices and data processing, the authors of this reference have substituted the classical film hologram by direct interferometric detection, proposing an electronic area sensor. The restoration of the tooth to its original shape was assumed to be highly automated, drawing from a knowledge base stored in the form of a tooth feature library.
Another early description of a dental fitting process is contained in U.S. Pat. No. 3,861,044 by William E. Swinson, Jr. This method teaches the recording of the three-dimensional shape of the cavity by conventional camera means. This process does not seem feasible, and no further reference to this work has been found.
An article by E. Dianne Rekow and Arthur G. Erdman ("Computer-aided Automatic Production of Dental Restorations" Proc. 4th meeting Europ. Soc. of Biomechanics, Davos 1986, p 117 ff.) lists the most promising candidates for the 3D acquisition. The authors chose the method of taking a stereo-pair by means of instant photography and analysing the stereo-pair in a central lab facility, which would also produce the desired piece to be sent back to the dental office to be seated in the tooth cavity during a second session.
Yet another scheme has been devised by Dr. Paul Heitlinger (German Offenlegungsschrift No. 29 36 847 Al, "Method and Apparatus for the Fabrication of Dental Prosthetic Work"). This technique uses a cast replica of the preparation which is recorded stereo-optically. The record is then computer-analysed and processed.
Finally, reference is made to the Inventors' own work, namely European Published Application No. 054 785 and corresponding U.S. Pat. No. 4,575,805. The method taught therein records the data by projecting a ruling onto the preparation and using active triangulation for the determination of depth. This procedure has been successfully utilized on a large number of patients
Since the state described in the Inventors' above-mentioned patent, considerable improvements have been added to the basic concepts, and are the subject of the present invention.
Experience has shown that while depth registration alone is adequate, it is not satisfactory to generate reconstructions with the degree of accuracy which is desirable under real-life conditions.
Before commencing a detailed description of the improvements, it is helpful to summarize the operation of a system built according to the technology known to
(1) The dentist carries out a three-dimensional scanning or registering of a prepared cavity directly in the patient's mouth. PA1 (2) The recorded three-dimensional data can be visually displayed in two fashions, either as a brightness coded profile (where points closer to the observer appear brighter than those further away) or as a perspective view such as is widely used in the art of CAD (Computer-Aided-Design). PA1 (3) Based on this representation of the shape the preparation, a manual or automatic reconstruction takes place to produce data utilized to control a milling machine which fabricates the restorative part. Preferably, the entire setup is located in the dental office, allowing restoration of one or several teeth in a single session.